Though earthquakes are rare here, Md. geologists are wary of the effects of possible future fracking.

Given that seismic activity is rare in the ancient rock of the Appalachians — and damaging earthquakes even rarer — there is only a single apparatus measuring underground rumblings within Maryland borders. But geologists are about to put another ear to the ground.

The Maryland Geological Survey, anticipating the possibility that hydraulic fracturing, or "fracking," for natural gas in the Marcellus shale deposits could increase seismic activity, plans to install a seismometer in Western Maryland.

Geologists want to gather more data on natural seismic activity before a state moratorium on hydraulic fracturing ends in 2017 and what are known as "induced" earthquakes might begin.

Fracking itself has not been linked to the swarms of earthquakes that have erupted in states such as Oklahoma. Geologists blame a process that disposes of briny water and other oil and gas extraction byproducts in deep wells. Maryland's geology is not considered suitable for those wells, though there are many of them in West Virginia and Pennsylvania.

But the risk and uncertainty here are great enough that scientists want to know more. Geologists know relatively little about faults beneath the region, which have produced recent earthquakes such as one that rattled Anne Arundel County this month and another that damaged historic buildings across the Mid-Atlantic in 2011. Another sensor should help reveal more about formations hundreds of millions of years old beneath the eastern United States.

"It's like a grocery store. You'd like to have a grocery store nearby so that everyone has a grocery store," said David Vanko, dean of Towson University's Fisher College of Science and Mathematics and chairman of the state's Marcellus Shale Safe Drilling Initiative Advisory Commission. "It would provide a data point that would be very useful to seismologists."

State geologists' lone permanent seismometer is buried in the Soldiers Delight Natural Historic Area in Owings Mills. A canister about 11/2 feet high and 8 inches thick, the seismometer sits on a concrete pad in direct contact with bedrock and uses a suspended weight to detect vibrations.

The spot was chosen because it is far from traffic and even trees swaying in the wind, noises that can interfere with readings.

Geologists will soon start looking for a similar location in Garrett or Allegany counties, said Richard Ortt, director of the Maryland Geological Survey. The agency, part of the state's Department of Natural Resources, is spending about $30,000 to buy a device that officials plan to install by June, he said.

A network of seismic sensors is important because multiple points are needed to pinpoint earthquake epicenters and intensity. While the U.S. Geological Survey has a network of transportable seismographs across the country, moving them gradually from west to east, devices permanently installed in the ground are spread more sparsely.

A nationwide array of seismometers gathers measurements from permanent sites near Morgantown, W.Va., Fredericksburg, Va., and State College, Pa. Ortt said state geologists also get data from other devices in those states and in Delaware.

"It's a network," Ortt said. "One station by itself gives you one piece of information."

When a small earthquake occurred Aug. 8 near Crownsville, its depth was initially estimated at 3 miles and some sources suggested its magnitude was 2.4. Geologists later revised those figures to a depth of 6.8 miles and a magnitude of 2.2.

Most of the time, the network is quiet. The bedrock beneath the Mid-Atlantic region is relatively stable because it lies at the center of a tectonic plate and has settled over hundreds of millions of years.

Earthquake-prone California, in contrast, sits at the edges of the Pacific and North American plates along the 28 million-year-old San Andreas fault.

The Mid-Atlantic got its biggest shake in decades four years ago this Sunday, when a 5.8-magnitude quake centered near Mineral, Va., shook the East Coast. It was Virginia's most intense earthquake since 1897.

There have been four quakes centered in Maryland over the past decade. In addition to this month's quake, there was a 3.4-magnitude temblor centered near Germantown in 2010, a 2.2-magnitude earthquake centered near the border of Frederick and Carroll counties in 2012, and a 1.5-magnitude quake centered at the northwest corner of Baltimore in 2007.

Geologists wonder whether their sensors will soon detect more activity. The General Assembly approved a bill in April outlawing fracking until October 2017. It became law without Gov. Larry Hogan's signature.

Fracking involves the high-pressure injection of water, chemicals and sand into wells to create cracks through which natural gas is released. In this region, the wells target a layer of shale known as the Marcellus formation, which stretches from Ontario, Canada, to Virginia, and lies beneath Garrett County and parts of Allegany County.

Blasts from the process itself cause what are known as micro-earthquakes, according to the U.S. Geological Survey. The mini-tremors, similar to those generated from activity at quarries, are rarely felt, the state reported last year.

Still, earthquakes have spiked in parts of the country that use deep wells to dispose of wastewater from fracking and other oil and gas drilling. Along with the fuel comes salty water and other liquids that are often deposited back into the ground, and U.S. Geological Survey researchers have found that can change the pressure along or against faults, causing them to slip.

The wells have been linked to a rise in earthquakes in Oklahoma, from 109 of at least magnitude 3 in 2013 to 585 last year. In Ohio, scientists linked a spate of 77 earthquakes in March 2014 to two oil and gas operations, prompting the state to halt activity at those sites.

There are no disposal wells in Maryland, Vanko said, and gas companies are not expected to request permission for any. But there were more than 1,800 in Pennsylvania, 750 in West Virginia and a dozen in Virginia as of 2013, according to the Environmental Protection Agency.

Drew Cobbs, executive director of the Maryland Petroleum Council, said he does not know of plans for any such wells in Maryland. And oil and gas companies are looking increasingly to treat or recycle the fluids, he said, for environmental and cost-saving reasons.

He said he thinks efforts to boost monitoring of seismic activity are premature.

"If there was something really going on or we were moving forward at this time with development of natural gas resources, it would make some sense," Cobbs said. "It looks like we're a few years away from that possibility."

Eric Robison, co-founder of Save Western Maryland, a group that formed to oppose wind projects but has expressed concerns about mining and shale gas, said he appreciates the caution, noting a series of inactive faults that the geological survey is mapping beneath the town of Accident and near a natural gas storage facility there.

"I think we should be very cognizant of what's happening with our geology before we jump in," Robison said.

Even if earthquake fears are unfounded, geologists said, there is a benefit to expanding the network of seismic sensors.

"It's difficult to say earthquake activity has increased if you don't have anything measuring it beforehand," Ortt said. "It's important to get this up and running so we can determine baseline seismic activity."